Fundamental knowledge investigations of soot-lubricant interactions continue. In earlier work [1-2], we examined the impact of formulation variables, engine type and mode of engine operation on the formation and nature of diesel soot and its interactions with the crankcase lubricant. Three types of North American heavy duty diesel engines were utilized: Mack EM6-285, GM 6.2L and GM 6.5L. Experiments identified additive compositions capable of providing good viscosity and wear control. Furthermore, we identified soot agglomeration, rather than amount of soot, as the phenomenon responsible for roller follower wear at low dispersant levels. Oil thickening results from the level of soot contamination, in combination with the “state” of the soot. The latter is noticeably affected by the lubricant dispersant level.Part 4 of our studies examines the impact of oil composition on a fluid's ability to handle soot in the Mack T-8 Test. A statistically designed experiment was used to examine the effects of dispersant level, dispersant type, antioxidant level, and detergent metal type on viscosity growth and other selected responses.Our studies identified dispersant type and level as key factors in controlling viscosity increase, pentane insolubles, size of soot particles and stability of soot particles in the used oil. We found that oil thickening has a complex dependence on soot, oil composition and oil sump temperature. No oil oxidation products were detected. We also examined the effect of our matrix factors on rocker cover sludge, oil consumption, filter plugging and filter mass, as well as on TBN decrease and wear metal contamination of the lubricants.